Glycan Diversity and Anti-Glycan Antibodies as Barriers to Gene Flow Mammalian cells are covered with a dense layer of sugar chains (glycans). Patterns of glycan diversity exist between cell types, between individuals, and between species. The evolutionary mechanisms underlying such patterns remain unknown but protective functions of glycan diversity have been proposed, given that non-self glycans are targets of innate and adaptive immunity. Mammalian glycans commonly terminate in sugars called sialic acids (Sias). The two most common Sias are N-Acetylneuraminic acid (Neu5Ac) and N- Glycolylneuraminic acid (Neu5Gc). Humans have lost the ability to produce Neu5Gc from its precursor Neu5Ac due to a gene inactivation. Humans together with all other old world monkeys (OWM) and apes have lost the ability to make another common glycan, the ?-Gal epitope, due to another mutation. The molecular cell surface landscape of each human cell thus differs from those of most other mammals by lacking millions of ?-Gal molecules and tens of millions of Neu5Gc molecules. Humans also make antibodies to both of these xenoglycans but how we get immunized remains unknown. This project will test the effect of these two xenoglycans on different forms of gene flow, viral infection and reproduction. Humans are unique among African primates for lacking endemic infection and ancient coevolution with two different enveloped retroviruses, immune deficiency viruses (SIV/HIV) and foamy viruses (SFV) until very recently. Glycans on enveloped viruses or sperm and fetal tissues can be targeted by the immune system. We propose that natural selection by glycan-binding pathogens may underlie initial glycan diversification, but that sexual selection in the form reproductive incompatibility may lead to fixation of glycan repertoires between species. In the process, these glycan differences provide partial protection from cross-species transmission of enveloped viruses. We will utilize gene knockout mice and mouse cell lines to test the effect of glycan mismatch and anti-glycan antibodies on both, viral and reproductive gene flow. We will also test the effect of anti-glycan antibodies from human sera for their neutralizing effects on viruses produced in Old and New World monkey cell lines which naturally differ in their xenoglycan profiles. This project promises important insights into the mechanism of poorly understood glycan evolution, reproductive compatibility and determinants of cross-species viral infections.